[avalanche] Add the concept of inconclusive votes

Summary:
Any error code starting with a the bit 1 will be considered as an inconclusive vote. The node is not for or against.

Depends on D2148

Test Plan: Added test cases to make sure this is handled properly.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2160